1. Field of the Invention
This invention relates generally to digital signal processing and digital networks, and more specifically to distribution of signals over a digital cable television network.
2. Description of the Related Art
There is a growing demand for a cable television (CATV) network to support a wide variety of services: analog video, digital video, interactive video, high-speed data access, telephony, and telemetry. Bundled services, supplying multiple services simultaneously, are desired. In order to meet the demand, the CATV network must be able to offer high signal quality over long distances, offer flexibility in adding or dropping services, provide network reliability, and provide cost efficiency.
Presently, information gathering equipment resides in a headend. Equipment used to process the gathered information and configure the information for reception by subscribers also resides in the headend. In a typical CATV network, information from various sources, including satellite or video feed, is received at the headend for broadcast in the CATV network. The information received may be legacy analog video channels operating at an Intermediate Frequency (IF) or digitally encoded video channels (e.g., Moving Picture Experts Group (MPEG) data). CATV broadcast signals are transmitted from the headend to subscribers in an analog format over a designated frequency bandwidth. A transmitter at the headend frequency-division multiplexes the video channels before broadcasting to multiple nodes. Each analog video channel is modulated onto its designated radio frequency carrier. The digital bitstream of each digital video channel is error-encoded, modulated, and converted to an analog signal before modulation onto its designated radio frequency carrier.
The analog nature of the broadcast signal limits the transmission distance from the headend to the nodes being served. The CATV network is typically a Hybrid-Fiber-Coax (HFC) system. The broadcast signal is often transmitted from the headend to the nodes using fiber optic cables. The broadcast signal is transmitted from the node to subscribers using coaxial cables. The quality of the analog signal can be sufficiently maintained in the range of 65 kilometers of fiber optic cable. Inherent non-linear characteristics, transmission of multiple channels simultaneously, and noise generated throughout the CATV network significantly degrade the analog signal beyond the 65 kilometers range limit.
An alternate architecture for the CATV network is a Multiplexed Fiber Passive Coax (MFPC) system. In the MFPC system, the broadcast signal is first transmitted from the headend to mux fiber nodes. The broadcast signal is then transmitted from the mux fiber nodes to mini fiber nodes. Both transmissions use fiber optic cables. The broadcast signal is transmitted from the mini fiber nodes to subscribers using coaxial cables. The mini fiber nodes function similarly to the nodes in the HFC system. However, each node typically services a heavier load (e.g., 500 to 2000 subscribers) in comparison to each mini fiber node (e.g., 50 to 80 subscribers). The MFPC system is an improvement over the HFC system. The MFPC system uses shorter coaxial cables to transmit signals from the fiber system to the subscriber. Shorter coaxial cables result in increased bandwidth capacity. Amplifiers in the coaxial cable transmission path are eliminated. Power can be delivered to subscriber equipment via the coaxial cables.
The present CATV network, using either the HFC or the MFPC system, is an open-loop system. The broadcast signals in an analog format are sent from the headend to the nodes, which in turn send the signals to the subscribers. The quality of the signal is not known until it reaches the subscriber. Errors caused by distortion, noise, or faulty equipment are not automatically monitored. The current CATV network is 95% reliable. However, interactive services require 99.9% reliability.
The present invention solves these and other problems by providing a cost-effective and flexible digital CATV network wherein a headend transmitter receives signals and produces a digital signal in a digital format and a node transmitter receives the digital signal in the digital format and produces an output in an analog format. In the existing CATV networks, signals are transmitted in the analog format.
In the digital CATV network, video signals are in a digital format for transmission from a headend to nodes in a cable distribution system. The nodes convert the digital data to an analog format for distribution to subscribers. Subscribers include homes, schools, businesses, and government agencies. In this application, the term home is synonymous with the term subscriber. The digital CATV network drastically improves signal quality as transmission of digital signals do not require a highly linear network. Digital signals can tolerate higher noise levels than analog signals. The quality of digital signals can be sufficiently maintained in transmission through thousands of kilometers of fiber optic cable by spacing repeaters or optical amplifiers in the transmission path (e.g., every 100 kilometers) to relay the digital signals.
In one embodiment, a digital transmitter at a headend digitizes each analog video channel and frames the digital data into a Synchronous Optical NETwork (SONET) Optical Carrier level 3c (OC-3c) bitstream. The electrical equivalent of OC-N is Synchronous Transport Signal level N (STS-N). In this application, the terms OC and STS are used interchangeably. OC-3c is sufficient to transmit a 6 MHz analog video channel with a reasonable signal-to-noise ratio. The digital headend transmitter also provides error-encoding to each digital video channel and frames the error-encoded digital video channels in groups of three into a SONET OC-3 bitstream. High quality digital video can be transmitted at an OC-1 bit-rate. N digital video channels can be framed into an OC-N bit-rate. SONET bitstreams from M analog video channels and groups of digital video channels are time-division multiplexed and sent at M times the OC-3 bit-rate through fiber optic cables from the headend to the nodes. In a MFPC system, the data is first broadcast from the headend to the mux fiber nodes which further broadcast the data to the mini fiber nodes. The mux-fiber nodes do not change the format of the data.
The SONET bitstreams are demultiplexed at the nodes back to the OC-3 bit-rate and deframed to recover the digital data. Digital data corresponding to analog video channels is converted back to an analog format. Digital data corresponding to digital video channels is digitally modulated and converted to an analog format. Channels in their analog format are frequency-division multiplexed by modulation onto designated radio frequency carriers and distributed through coaxial cables to homes.
Information for interactive services, such as telephony or the Internet, originates from many locations and is not consistently transmitted over time. Telephone calls are typically short in duration, averaging about 3 minutes. Internet traffic duration averages over 30 minutes. Therefore, the ability to add or drop channels easily is advantageous. The digital CATV network time-division multiplexes channels for transmission from the headend to the nodes. Time Division Multiplexing (TDM) allows for multiple locations from the headend to the nodes where channels can be easily added or dropped as the need arises. Telephony and Internet services are already built on the characteristics and performance of TDM technology.
Interactive services make the CATV network increasingly more symmetric, with as much information traveling upstream as downstream. Downstream refers to data that flows from the CATV network to the homes, and upstream refers to data that flows from the homes to the CATV network. In one embodiment, bandwidth for upstream data is allocated between 5 MHz and 45 MHz as well as between 900 MHz and 1 GHz. Each headend serves 10,000 to 300,000 or more homes. Each node serves a subset of the homes served by the headend. It is advantageous to be able to add or drop data at each node so that fewer homes share the allocated bandwidth for upstream data.
A location where data can be added or dropped is referred to as a xe2x80x9cPoint of Presencexe2x80x9d (POP). A POP links external data networks, including the Internet, cellular network, Public Switched Telephone Network (PSTN), and satellite network, to the digital CATV network. Information from the external data networks passes to the digital CATV network at the POP. Additionally, information from the digital CATV network can pass to the external data networks at the POP. For example, the headend or the node can serve as a POP. A bank of modems can be incorporated in each POP to interface between the external data networks and the homes. The bank of modems can also pass information between the homes serviced by the digital CATV network. Other locations in the digital CATV network, such as the mux fiber node in the MFPC system, can also serve as a POP. Multiple POPs between the headend and the nodes provide the flexibility to add or drop data that is common to multiple nodes.
A closed-loop digital CATV network increases the reliability of the network due to feedback. Digital format includes extra bits, such as parity bits, to detect defects, errors, or failures in transmission. Remote indications control action in network protocols and bad packets can be resent without interruption.
A digital CATV network is cost-efficient. Costly processing, such as Forward Error Correction (FEC) of digital video channels, is performed at a headend. Standardized, thus economical, digital network equipment is used throughout the network by framing digital data into standardized bit-rates, such as OC-3, OC-12, OC48, or OC-192. The ability to add or drop channels at nodes increases the effective upstream bandwidth without installing more fiber optic cables from the headend to the nodes.